gpui: Remove some unnecessary unsafe code (#40483)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/gpui/src/app/entity_map.rs

@@ -378,11 +378,9 @@ pub struct Entity<T> {
     #[deref]
     #[deref_mut]
     pub(crate) any_entity: AnyEntity,
-    pub(crate) entity_type: PhantomData<T>,
+    pub(crate) entity_type: PhantomData<fn(T) -> T>,
 }
 
-unsafe impl<T> Send for Entity<T> {}
-unsafe impl<T> Sync for Entity<T> {}
 impl<T> Sealed for Entity<T> {}
 
 impl<T: 'static> Entity<T> {
@@ -657,7 +655,7 @@ pub struct WeakEntity<T> {
     #[deref]
     #[deref_mut]
     any_entity: AnyWeakEntity,
-    entity_type: PhantomData<T>,
+    entity_type: PhantomData<fn(T) -> T>,
 }
 
 impl<T> std::fmt::Debug for WeakEntity<T> {
@@ -669,9 +667,6 @@ impl<T> std::fmt::Debug for WeakEntity<T> {
     }
 }
 
-unsafe impl<T> Send for WeakEntity<T> {}
-unsafe impl<T> Sync for WeakEntity<T> {}
-
 impl<T> Clone for WeakEntity<T> {
     fn clone(&self) -> Self {
         Self {

crates/gpui/src/app/test_context.rs

@@ -888,7 +888,9 @@ impl VisualTestContext {
         // safety: on_quit will be called after the test has finished.
         // the executor will ensure that all tasks related to the test have stopped.
         // so there is no way for cx to be accessed after on_quit is called.
-        let cx = Box::leak(unsafe { Box::from_raw(ptr) });
+        // todo: This is unsound under stacked borrows (also tree borrows probably?)
+        // the mutable reference invalidates `ptr` which is later used in the closure
+        let cx = unsafe { &mut *ptr };
         cx.on_quit(move || unsafe {
             drop(Box::from_raw(ptr));
         });